Monitoring circuit for a high-intensity glow discharge for metallurgical processes

ABSTRACT

Monitoring circuit for a glow discharge wherein the voltage and current of the discharge are compared by the circuit and a Schmitt trigger generates a control signal if the voltage-current relationship indicates unsafe operation.

United States Patent:

Inventor MONITORING CIRCUIT FOR A HIGH-INTENSITY GLOW DISCHARGE FOR METALLURGICAL PROCESSES 2 Claims, 3 Drawing Figs.

11.8. CI .Q 315/127, 324/24, 204/164, 317/43 Int. Cl ll02h 7/00 Field ofSearch 315/119,

127, 136; 307/247, 290, 297, 235; 328/146, 161, 194-196, 198, 207; 3l7/l48.5,3l,43;

324/24; 323/4, 9, 20; 204/164, 177, 192, 312, (lnquired); 340/248, 253

[56] References Cited UNITED STATES PATENTS 2,955,237 10/1960 Wyndham 317/1485 3,300,689 1 l 967 Beddoes 3 l 7/ 1 48.5

3,303,412 2/ 1967 Gately 323/4 3,339,111 8/1967 Possner.... 317/16 3,017,564 l/l962 Barney 323/9 3,355,626 1 H1967 Schmidt 315/241 OTHER REFERENCES INDUSTRIAL ELECTRONICS AND CONTROLS, by Royce Gerald (John Wiley & Sons, New York), 2nd Edition, 1960 pages: Title Page, Page IV, Page 176; page 224.

Primary Examiner-James D. Kallam Assistant Examiner-W. Larkins Attorney-Leo A. Rosetta 1 ABSTRACT: Monitoring circuit for a glow discharge wherein the voltage and current of the discharge are compared by the circuit and a Schmitt trigger generates a control signal if the voltage-current relationship indicates unsafe operation.

D.C.GURRENT GLOW SOURCE lg Rs CHAMBER scumrr 5 TRIGGER Patented May 18, 1971 2 Sheets-Sheet 1 INVENTOP Gau flaw/v 6x 1556144 BY A T'T'OPNE' K5 Patented May 18, 1971 3,579,029

2 Sheets-Sheet 2 Fig. 3

D.C.GURRENT GLOW SOURCE lg Rs CHAMBER SCHMITT 5 TRIGGER M/VE'NTOE GELL/ 4W0 \SPESCHH ATTORNEYS Glow-discharge processes for the treatment of workpieces in a gas atmosphere, e.g. hardening processes, surface improvement by gas diffusion or cathode sputtering, are well known. In this context it is highly important for the glow discharge to burn evenly during the frequently extended period of treatment without changing into arc discharges which leave burning spots of varying sizes.

This object is commonly achieved by influencing the electrical supply circuit for the glow discharge, by way of example by brief reductions of the operating voltage or by incorporation of an impedance. Such influencing is performed by suita control means such as electronic circuits controlled by signals which may, by way of example, be given when the operating current of the glow discharge exceeds a certain rated value. However, it has been found to be difficultso to predetermine this rated value in the operation of a glow discharge employed for metallurgical purposes that economical operation is ensured on the one hand and, on the other, that the supply circuit is influenced dependably and fast enough when a disturbance in the discharge occurs. These difficulties result mainly from the fact that the voltage/current characteristic of such a glow discharge is not a straight line but runs as exemplified in FIG. 1.

The present invention eliminates these difficulties and relates to a method of monitoring the electrical behavior of a high-intensity glow discharge for metallurgical processes such as diffusion processes, cathode sputtering, hardening by nitriding and the like in order to influence the supply circuit by signal-controlled means. It is characterized by the fact that an unsafe area is defined by an adjustable sensor which can be influenced by the supply circuits below the desired rated curve for the operating point of the glow discharge in the voltage/current characteristic family, the said area also comprising the field of unstable discharges and the said sensor supplying a control signal for the said means when the operating point deviates from the rated curve to the extent that it lies within the unsafe area.

The invention further relates to a device for the performance of the said method. This device is characterized by a sensor consisting of a PNP transistor circuit and a Schmitt trigger of a design known per se, by a series resistance in the supply circuit to which an input terminal of the Schmitt trigger and the collector of the transistor are connected via a barrier resistance, by a voltage divider parallel with the voltage source of which the tap is connected to the base of the transistor, and by a series resistance and a bias in the emitter circuit of the transistor, all being arranged in such a manner that the voltage of the resistance in the supply circuit which depends on the discharge current and a countervoltage depending on the collector current through the barrier resistance are applied to the trigger input terminals, the collector current being so adjusted by means of the partial voltage tapped at the voltage divider and the emitter bias that the response voltage at the trigger input terminals is exceeded only for values of the discharge current and the discharge voltage within the unsafe area of the I 4 voltage/current characteristic family of the glow discharge.

An embodiment of this invention is described in greater detail with reference to the drawing in which:

FIG. 1 is a diagram of the voltage/current characteristic family in the operation of an electrical glow discharge, and

FIG. 2 is a sensor circuit for the performance of the present method, and

FIG. 3 is a block-type diagram illustrating the invention as applied to a three-phase power source.

The process here disclosed relates to an adjustable safety area located directly below the effective characteristic of the glow discharge, cf. FIG. l.-- The current and voltage are continuously measured and the position of the operating point thus determined assessed. If the operating point falls within the predetermined unsafe area, a quick-acting switch responds, i.e. it triggers the contemplated protective measures 2 in the supply circuit. In general, the latter consist in the known manner in a short discontinuation of the supply voltage and, in the event of frequent repetitions of the disturbance, in complete disconnection of the voltage source. Adjustment of the safety line is most commonly effected by two potentiometers. The first is designed to adjust the minimum burning voltage; the other, to adjust the gradient dUldI. These two adjustments should be adapted depending on the'type of gas, pressure, temperature and cathode surface of the discharge. Changes in the operating voltage require no adjustment of the quick-acting switch which is suitableeveri for extreme impulse operation of the glow discharge.

This quick-acting switch principle is also suitable for threephase current. Three similar sensor circuits are employed, one for each phase, and the three outlets arranged in parallel. The safety means are then triggered if a disturbance is detected in any one of the three phases. In the device here disclosed it is assumed that the transformer neutral point is not connected and, respectively, the secondary side of the transformer is connected in delta circuit. The three symmetrical cathodes are each connected to one phase. Current measurement in eachphase raises no problems. Schematic FIG. 3 shows how the invention is adapted to a three-phase power source 1 and wherein each line of the three-phase current is connected to a complete sensing transistor and associated circuitry indicated by the blocks 20. The circuitry within each block 20 is the same as that appearing within the dotted rectangle 20 of FIG. 2. Measurement of the proper burning voltage is complex since the discharge is operative alternatively between the three cathodes. It has been found that the gas plasma is approximately at the most positive potential of the three cathodes. Accordingly, at least one of the electrodes is always cathodic and the voltage between it and the positive plasma is decisive for its discharge. The metallic anode assuming approximately the potential of the plasma, has proved to be satisfactory to measure the voltage between each electrode and the anode. The measuring arrangement of each phase is thus fomied by measuring the current of the phase involved and the voltage of the respective electrode against the anode, it being sufficient to record the negative values only. Disturbances occur mostly at the cathodically glowing electrodes and every disturbance is thus recorded since the discharge operates without o-lead so that any disturbing current must pass a measuring point.

FIG. 2 shows an example of a simple circuit with only one transistor in the actual sensing stage. The reference numeral 1 designates the supply rectifier, 2 the discharge anode or glow chamber in which the glow discharge occurs. The current in the glow discharge I, causes a voltage drop across resistance 6. This voltage is applied, via the resistance 7, to the input side of a Schmitt trigger 4. This Schmitt trigger of known design responds if its input voltage U, reaches a certain value and then produces a signal at output 5. Applied to the discharge anode 2 is a voltage U, so that the portion aU, is supplied, via

the voltage divider 9, 10, to the base of the transistor 3; This transistor forms the essential portion of the sensing circuit.

The voltage all, applies a bias on the transistor base resulting in a current through the emitter and the collector of thetransistor which essentially depends on aU,,, U and the resistance 8. The voltage drop in the resistance 7 caused by this current counteracts the voltage caused by current 1, and thus allows the Schmitt trigger 4 to respond only in the presence of an accordingly greater current I,,. The safety line resulting,

from this arrangement and others, which delimits the safety area, is shown in FIG. 1. The reference numeral 1 designates the real characteristic of the glow discharge, 2 the safety line obtained by an arrangement as per FIG. 2, and the hatched I claim:

1. Means for monitoring the electrical behavior of a high-intensity glow discharge apparatus having an input circuit means, comprising: a PNP transistor and a Schmitt trigger, a first resistor in series in said input circuit, the input to said Schmitt trigger and the collector of said transistor being connected to one end of said first resistor through a second resistor; a voltage divider across said input circuit having a point therein connected to the base of said transistor; second circuit means including a source of bias voltage for applying a potential between the emitter and base of said transistor, said input circuit means and second circuit means cooperating to produce a signal in response to the voltage across said first resistor and a counter voltage dependent on the current through the collector of said transistor and said second resistor, said signal being applied to the trigger input, said voltage divider including means for varying said collector current whereby the response voltage at the trigger input is exceeded only for values of the discharge current and the discharge voltage within the range of values of the voltage-current characteristic family of the glow discharge.

2. Means as defined in claim 1 wherein said input circuit comprises a three-phase circuit there being a transistor connected to each leg of said three-phase circuit in the manner described; there being a single Schmitt trigger connected to all said transistors whereby the input to said trigger is the algebraic sum of the phase voltages and the three counter voltages. 

1. Means for monitoring the electrical behavior of a highintensity glow discharge apparatus having an input circuit means, comprising: a PNP transistor and a Schmitt trigger, a first resistor in series in said input circuit, the input to said Schmitt trigger and the collector of said transistor being connected to one end of said first resistor through a second resistor; a voltage divider across said input circuit having a point therein connected to the base of said transistor; second circuit means including a source of bias voltage for applying a potential between the emitter and base of said transistor, said input circuit means and second circuit means cooperating to produce a signal in response to the voltage across said first resistor and a counter voltage dependent on the current through the collector of said transistor and said second resistor, said signal being applied to the trigger input, said voltage divider including means for varying said collector current whereby the response voltage at the trigger input is exceeded only for values of the discharge current and the discharge voltage within the range of values of the voltage-current characteristic family of the glow discharge.
 2. Means as defined in claim 1 wherein said input circuit comprises a three-phase circuit there being a transistor connected to each leg of said three-phase circuit in the manner described; there being a single Schmitt trigger connected to all said transistors whereby the input to said trigger is the algebraic sum of the phase voltages and the three counter voltages. 